A conventional wastewater treatment plant typically involves aeration. One type of aeration device is a fine bubble diffuser. The most common types of fine bubble diffusers are ceramic elements and rubber membrane diffusers. In some wastewater treatment systems, the systems specify the use of both ceramic element diffusers and rubber membrane diffusers in the same system. For example, ceramic elements may be installed throughout most of the treatment tanks in the wastewater treatment system and with the use of rubber membrane diffusers installed in the remainder of the system. In such wastewater treatment systems, the rubber membrane diffusers and the ceramic elements are not mounted on any single header but are instead installed on separate grids of headers. However, both sets of grids of headers share the same air supply. In some applications, valves cannot be used to balance the air flow between the grid supporting the ceramic element diffusers and the grid supporting the rubber membrane diffusers.
It is known from testing that operating the diffusers for long periods of time results in headloss rise or “headloss migration” due to a change in the relative resistance to air flow through the fine bubble diffusers caused primarily by either biofouling and/or changes in water absorption of the diffuser elements.
In the design of a wastewater system it is difficult to design membrane and ceramic element diffusers to have identical headloss versus air flow curves. Additionally, it is also difficult to maintain the headloss from element to element within manufacturing control limits and to influence or predict the different rates of headloss migration arising as a result of fouling of the diffuser elements and changes in surface properties of the rubber membrane diffusers. It is also difficult to predict the changes over time of the flow volume into the wastewater treatment system and the operating flow rates of wastewater treated by the system as additional demands are placed on the system by growth of a community.
When two types of diffusers such as ceramic diffusers and rubber membrane diffusers are installed on headers or grids connected to the same air supply system, the less restrictive elements produce higher air flows. Even if balancing orifices are used to compensate for air flow between the header pipes into the diffuser elements, the balancing orifices have a non-linear headloss versus air flow curve, and that non-linear headloss versus air flow curve is significantly different than the headloss/air flow curve of the diffuser elements. Because the headloss/air flow curve of a ceramic diffuser element does not match the headloss/air flow curve of a rubber membrane diffuser, changes in air flow through the headers can only truly be balanced at one flow rate. Additionally, different rates of headloss migration over time complicate the situation further. If the migration rates are significantly different, which is usually the case between ceramic diffusers and rubber membrane diffusers, the balanced flow point will shift with time to a different system flow rate.
All of these factors prevent the engineer designing a system that may be balanced at initiation of operation of the wastewater treatment system to maintain its balance over the life of the system's operation.